ADAPTIVE ANTENNAS. TYPES OF BEAMFORMING

Transcription

1 ADAPTIVE ANTENNAS TYPES OF BEAMFORMING 1

2 1- Outlines This chapter will introduce : Essential terminologies for beamforming; BF Demonstrating the function of the complex weights and how the phase and amplitude weights impact on the control of the array radiation pattern Types of BF

3 BF definition 2- BF terminologies Beam forming is a method used to create the radiation pattern of an array antenna by adding constructively the weights of the signals in the direction of SOI and nulling the pattern in the direction of SNOI (interference) This array can be antennas in the smart antennas context, or any other types of sensors (radars, medical sensors etc) can be an array of microphones in the speech signal processing context Beamforming can be used at both the transmitting and receiving ends in order to achieve spatial selectivity i.e., an appropriate feeding allows antenna arrays to steer their beam and nulls towards certain directions, this is often referred to as spatial filtering

4 BF network In transmission mode, the majority of signal energy transmitted from a group of sensor array can be directed in a chosen angular direction In reception mode, you can calibrate your group of sensors when receiving signals such that you predominantly receive from a chosen angular direction TX RX BF network

5 Weight vector Is a vector of complex weights w, each element consists of real and imaginary components, or alternatively, amplitude and phase components w m e w m is the complex weight of the n th element, α m is the amplitude weight of the n th element and β m is the phase weight of the n th element -Amplitude components control the sidelobe level and main beam width -Phase components control the angle of the main beam and nulls Phase weights for narrowband arrays are applied by a phase shifter Steering vector If there are K transmitters, K received signal vectors can be determined as a radar tracking multiple targets, or in a mobile communications system where multiple users are active. The received signal vector of the d th signal is frequently referred to as the steering vector a(θ d ) which is the array factor of any array, depends on the angle of arrival of each incident signal m j m 5

8 Null steering -The previous case will not produce a maximum SNR in the presence of directional interference. Such scenarios are very common in radar deployments where an intentional jammer may be targeting the area, as well as mobile communications where other network users will create unintentional interference -Thus null steering is useful when it is necessary to attenuate unwanted signals arriving at angles other than that of the main beam Let a o be the main beam steering vector (K=M-1) a 1,..., a K are k steering vectors for the K nulls W H = [w 1 w 2 w 3 w M ] a 1 e e j( M 1 jkd sin sin ) kd sin1 sin1 1 a k e e 1 jkd sin sin... j( M 1) kd sin sin k k k k 8

10 Example Compute the weight vector required to steer the main beam of a uniform two isotropic element array oriented in y-direction towards the required signal in the plane φ=90 o, at 45 o from the array boresight and to minimize the interfering signal that appears at - 10 o from the array boresight. Assume the element spacing d = λ/2. Such a scenario is typical of a TV receiver situated on a tall building and is therefore able to receive signals from both a local transmitter and a distant cochannel transmitter. A null is then steered towards the co-channel transmitter to provide attenuation of this unwanted signal Solution M=2, θ o =45 o,θ k=1 =θ 1 = -10 o, φ =90 o The electric field of the received signal is E n (, ) e n0 a T o = [ 1 e j2.221 ] a T 1 = [ 1 e -j ] A 1 1 j2.221 j e e C=[1 0] N 1 jnkdsin sin W H =CA -1 W H = [ (0.495 j0.096) ( j0.343) ] W= [ ( j0.096) ( j0.343) ] T E o (t) The null steering technique described here jointly steers the main beam and nulls to the desired angles. Modifying the vector C enables the existence of nulls and beams (or signal minima and maxima) to be specified according to the prevailing requirements θ E 1 (t) y

11 -When the number of required nulls is less than K, K= M 1, M is the number of array elements. Matrix A is not a square matrix (the matrix inversion will be singular and can not be inverted) -Under such conditions suitable weights may be given by: For a solution does not minimize the uncorrelated noise at the array output W H CA H ( AA H ) 1 For a formulation requires noise with variance σ n be added in the system W H CA H ( AA H 2 1 n I) 11

13 Onset of grating lobes So far the analysis of the behaviour of arrays has considered elements with spacing of d = λ/2. What happens when other element spacing are used? Such a scenario occurs when certain physical design constraints are imposed on the array size which impose to increase the spacing An array of two elements with unity amplitude and zero phase weights, oriented on y-axis, the radiation pattern for θ=90 o ; azimuth plane is shown for : d = 0.5λ d = 0.6λ d = 2.0λ The wider spacing the narrower the main lobe and the on setting of additional lobes (grating lobes) max sin 1 ( n / d) n 0,1,2,... 13

15 Amplitude weights The amplitude components of the weight vector control the sidelobe level and main lobe beam width An array of four element with zero-phase and different amplitude weights as: [1111] [ ] [ ] [0110] The more tapering along the array edges the wider the main lobe and the lower SLL It can be seem that the radiation pattern of the four element array approaches that of the two element array as the amplitude weights of the outside elements are reduced 15

16 Window functions Window functions enable the amplitude weights to be controlled with certain constraints. The constraints are related to the characteristics of the choice of function, such as desired sidelobe level or main beam width, or a combination of both (1)Rectangular window (3)Blackman window W(n)=1 (2)Bartlett window For odd N (4)Triangular window For odd N - For even N For even N 16

17 (5)Hamming window (6)Hanning window (7)Kaiser window I o is a modified Bessel function of order zero large α gives lower sidelobe levels at the expense of main lobe width This type of beam steering or BF is suitable for the case of fixed or switched beam, i.e., it is a conventional non adaptive narrowband BF 17

20 4- Types of beamforming Scanning RADAR either tracking or searching or both USER A W 1A 1 A -The beam is to be scanned across the aperture in which a radar system is searching for new targets. Once the scanning beamformer has acquired a target it will be handed to a conventional beamformer in order to track the new target. Scanning will then resume USER B W 1B W 2A W 2B 2 B -It is implemented by incrementing the weight vector W, mechanically or electronically, to steer the angle of the main beam across the array aperture. It could be either azimuth or elevation scanning or in both dimensions -It is not classified as adaptive because they follow a programmed sequence instead of dynamically adapting to a certain scenario. It is conventional array or conventional BF 20

21 4-Types of beamforming Scanning Phased array -It utilizes the DoA information concerning the desired user or target to steer the beam toward SOI -In mobile communication system, the direction of the mobile is created through weighting received signal by each antenna element ( in case of phased array antenna) or through the reflector antenna system. Then the beam is launched to communicate with the user -In this type of beamforming the amplitude of the weights are held constant while the only phases are changed -It is a conventional BF 21

23 Switched approach 4-Types of beamforming The SNR is measured for each beam and the maximum is determined. The beam associated with the largest SNR is chosen for further processing, i.e., at any time, all channels assigned to the cell that is serving by this array are available to all users. Therefore a particular beam, at certain times, may serve several users All channels are available to all beams in the cell More efficient and can serve up to N users (N is the number of available channels) but: -More complex to implement -Requires measurement of SNR, determine the maximum one and an RF switch should choose the appropriate beam -This process should be repeated for each user Butler matrix

24 Sectored approach If the mobile moves into the area covered by a different beam a handoff must occur 4-Types of beamforming It subdivides the sector into many narrow beams Each beam can be treated as an individual sector serving an individual user or a group of users Multiple antenna arrays are implemented It combines the outputs of multiple antennas in such a way as to form finely sectorized (directional) beams with more spatial selectivity than those could be achieved with conventional single element approaches The channels available in the cell are divided equally between the beams As a user (mobile, target, ) moves through the sector, the processor detects the signal strength and switch from one beam to another according to the strongest one. The beam is switched to communicate with the user Processor It suffers from greater number of handoffs

25 4-Types of beamforming Scanning Phased array Switched beam forming Butler matrix Bless array It is implemented using: 1- A number of fixed, independent, directional antennas 2- Antenna array and analogue beamformer such as Butler matrix (narrowband BF) or Bless matrix ( wideband BF ). Both matrices are explained in narrowband and wideband BF 3- Grid Of Beams (GOB) can be used with digital beamforming systems. It selects the best weights from a stored set of weights. It however, leads to a more complex implementation due to the drawbacks associated with digital beamforming. It does not require the beams to be orthogonal and an arbitrary number of beams can be formed Phased array is more smarter than switching beam because it generates more beams which could reach infinity 25

26 4- Types of beamforming Scanning Phased array Switched beam forming Adaptive beam forming Butler matrix Bless array An infinite number of patterns (scenario-based) that are adjusted in real time If the desired signal and interference occupy the same frequency band, then temporal filtering often cannot be used to separate signals from interference However, the desired and interfering signals often originate from different angles This spatial separation can be exploited to separate signals from interference using a spatial filter at the receiver A full adaptive beamformer is a processor used in conjunction with an array of antenna elements, to provide a versatile form of spatial filtering i.e., adaptive beamforming is able to automatically adapt the radiated power from an antenna array to different situations and communication scenarios 26

27 4- Types of beamforming Scanning Phased array Switched beam forming Adaptive beam forming Butler matrix Bless array Temporal reference Spatial reference Adaptive algorithm DOA algorithm The weights for such a spatial filter can be computed in the time (temporal reference beamforming) or spatial (spatial reference beamforming) domain or without the use of a reference signal, which is referred to as blind beamforming -Temporal reference BF: The weights are computed in time with training sequence (Weiner Hopf equations+ optimization algorithms MMSE, MSINR,) -Spatial reference BF: Does not use of embedded training sequences. DoA is used to steer the wanted signal and nulls directed toward the interference ( needs DoA algorithms, MUSIC, SPIRIT, 27

29 Switched scheme Select the strongest beam pattern Adaptive scheme The beam pattern is adjusted in real time according to: either a part of the desired signal is known through the training sequence ( Temporal BF ) or the transmitting DOA is set so that the beam is maximized toward the desired angle ( Spatial BF ) 29

30 Switched Low cost Easy to implement Comparison between Switched and Adaptive BF Integration Adaptive High cost Complex Less hardware redundancy Less Difficult to distinguish between the desired signal and interference Does not react to the movement of interference Coverage Interference rejection More Focusing is narrower Capable of nulling the interference signal

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